hépppp

`a)` Ta có : `A=123.123=123.(121+2)=123.121+123.2`

                 `B=121.124=121.(1+123)=121+121.123`

Vì `123.2>121 => 123.121+123.2>121+121.123 => A>B`

`b)` Ta có : `C=123.137137=137.123.1001`

                  `D=137.123123=137.123.1001`

Vì `137.123.1001=137.123.1001 => C=D`

`c)` Ta có : `E=2015.2017=2015.(2016+1)=2015.2016+2015`

                 `F=2016.2016=2016.(2015+1)=2015.2016+2016`

Vì `2016>2015 => 2015.2016+2016>2015.2016+2015 => F>E`